1. Field of Invention
The present invention relates to a light control sheet to direction-control an angularly extending light inputted from an oblique direction so that an output light is directed to an desired direction, also relating to a surface light source device and LCD using the light control sheet. The present invention is applicable to general light controls and various devices which require an illumination light extending like a face, for example, display incorporated in a personal computer or car navigation system.
2. Related Art
A proposed surface light source device employs a light guide plate made of a transparent light guiding material or light scattering-guiding material and a prism sheet, being broadly applied to various uses such as backlighting of liquid crystal display. A prism sheet is a most popular light control sheet which is used for direction-control an obliquely inputted light so that an output light is directed to an desired direction. A prism sheet is made of a light-permeable material having a prismatic surface provided with a great number of prism-like projection rows.
FIG. 1a is a partially exploded view illustration an outlined structure of a liquid crystal display provided with a backlighting arrangement in which a surface light source device of side light type employing a conventional and usual prism sheet, and FIG. 1b is a partially enlarged cross section view a part of the liquid crystal display. It is noted that thickness of a prism sheet 4 or other members, formation pitch an depth of prism elements and so forth are exaggerated for the sake of illustration.
Referring to the figures, reference numeral 1 denotes a light guide plate that is an optical member formed of transparent light guiding material or light scattering-guiding material and having a wedge-like cross section. Light scattering-guiding material is a well-known optical material having both light-guiding function and inner-scattering function, being made of, for instance, matrix of polymethyl methacrylate (PMMA) and xe2x80x9csubstance having a different refractive indexxe2x80x9d which is uniformly disposed in the matrix. It is noted that xe2x80x9csubstance having a different refractive indexxe2x80x9d is substance having a refractive index substantially different from that of the matrix.
A thicker end face of the light guide plate 1 provides an incidence end face 2 near to which a primary light source element (fluorescent lamp) L backed by a reflector R is disposed. The light guide plate 1 has major faces, one (front face) providing an emission face 5, the other (rear face) providing a back face 6. A reflector 3 is disposed along the back face 6. The reflector 3 is formed of silver foil with regular reflectivity or white sheet with diffusive reflectivity.
Ae known well, the emission face 5 emits an emission flux having a n obliquely-forwarding directivity. The prism sheet 4 is arranged on the outside of the emission face 5 so that a prismatic face (light input face) is directed inward.
Referring to the partially-exploded illustration, a light output face (outer face) 4c provides a flat surface. On the outside of the light output face 4c disposed is a liquid crystal display panel LP and a polarization separating sheet LS which is interposed between the light output face (outer face) 4c and the liquid crystal display panel LP. The liquid crystal display panel LP has a well-known structure such that elements such as liquid crystal cell and transparent electrodes are interposed between two polarizing plates arranged so that their polarization axes make the right angle with each other.
The polarization separating sheet LS is an optical element that tends to be used currently, being disposed between a polarizing plate on the inside of the liquid crystal display panel LP and the prism sheet 4. The polarization separating sheet LS has a property that shows a high transmissivity for a polarization component parallel to a polarization axis of the polarizing plate on the inside of the liquid crystal display panel LP and shows a high reflectivity for a polarization component perpendicular to the polarization axis of the polarizing plate on the inside of the liquid crystal display panel LP.
It is noted that a space (air layer), not shown, is formed between the liquid crystal display panel LP and the polarization separating sheet LS or between the liquid crystal display panel LP and the prism sheet 4 (if no polarization separating sheet is employed), as required, in order to prevent these members to be stuck to each other.
The prism sheet 4 has an input face provided by a prism surface having a great number of prism element rows. These prism element rows are orientated generally in a direction parallel to the incidence end face 2 of the light guide plate 1. As shown in a partially enlarged cross section FIG. 1b, each prism element row a pair of slopes 4a and 4b which provides a V-shaped groove.
Angle xcfx86a is defined as an inclination angle of a first slope 4a directed toward the side of the incidence end face 2 and xcfx86b is defined as an inclination angle of a second slope 4b directed to the opposite side. Angles xcfx86a and xcfx86b are measured with respect to a frontal direction (See reference N), respectively. A prism sheet that satisfies a condition, substantially xcfx86a=xcfx86b (0 degreexe2x89xa6xcfx86a less than 90 degrees), is called xe2x80x9csymmetric prism sheetxe2x80x9d and a prism sheet that satisfies a condition, substantially xcfx86axe2x89xa0xcfx86b, is called xe2x80x9casymmetric prism sheetxe2x80x9d. An asymmetric prism sheet is disclosed in WO98/40664.
Light is introduced into the light guide plate 1 from the light source element L, being guided toward a thinner side end face 7 with repeated reflections at the emission face 5 and back face 6. On the way of this travelling, illumination light is emitted from the emission face (front face) 5 gradually. The emission face 5 may have a matted surface or inner scattering may be utilized (if light scattering guide is employed) in order to promote emission.
As known well, the emission face 5 provides emission which shows a clear directivity to a direction inclined forward as a whole (Emission directivity of light guide plate). It is noted that such emission directivity may be relaxed if the emission face 5 or the back face 6 is provided with light diffusibility.
FIG. 2 is a graph showing angular intensity characteristics of emission from an emission face 5 of a typical light guide plate (transparent light guide plate having a matted emission face). In the graph, the abscissa shows directions in a plane perpendicular to the incidence end face 2. Angle indication of 0 degree denotes a frontal direction, and minus values correspond to the side of incidence end face 2 and plus values correspond to the side of distal end (forward side). The ordinate shows luminance in unit xe2x80x9ca.u.xe2x80x9d that is defined so that the peak value is just xe2x80x9c1.0xe2x80x9d.
As understood from this graph, the emission flux has a remarkably clear directivity such that a peak direction (emission angle of a main beam) is somewhat greater than 70 degrees. In general, angular position of peak direction may vary in a variation range from several degrees to ten and several degrees in dependence on factors such as size or material (transparent material or light scattering guide) of the light guide plate, properties of the emission (ability of light scattering) face and back face, and property of the reflection sheet disposed on the side of the back face. However, in general, there are no great difference among curves of graph provided in the respective cases.
FIG. 3 is a diagram illustrating a fundamental operation of the light control sheet 4, which is conventionally used, wherein it is provided that the light guide plate 1 has emission directivity of as described.
Referring to FIG. 3, the prism sheet 4 is disposed along the emission face 5 of the light guide plate 1 so that the prismatic surface is directed inward. Each prism element has a vertical angle, for example, xcfx86a+xcfx86b=about 66 degrees.
If the light guide plate of the above-mentioned example is employed and light is supplied from a direction of arrow L1, a propagation direction of a beam representing the emission flux from the emission face 5 gives an angle xcex82=about 73 degrees as understood from the above description. Considering that the light guide plate has a refractive index about 1.5, an incident angle xcex81 to the emission face 5 must be about 38 degrees for rendering xcex82 about 73 degrees. Hereafter in this specification, a flux of beams corresponding to a preferential propagation direction is called xe2x80x9cmain light fluxxe2x80x9d. A beam representing the propagation direction of such main light flux is called xe2x80x9cmain beamxe2x80x9d. In the illustration, a main beam is denoted by reference symbol S1.
The main beam S1 emitted from the emission face 5 is incident to a slope 4a, one of a pair of slopes, of the prism sheet 4 at a remarkably small incidence angle after travelling an air layer AR (refractive index n0=about 1.0) straight. There is actually little probability that the main beam is incident to the other slope 4b. 
Then the main beam S1 travels the inside straight as far as the slope 4b, being regularly reflected by the slope 4b. The beam regularly reflected is incident to a light output face 4c of the prism sheet 4 at a roughly right angle, being emitted from the prism sheet 4.
On the way of this process, the main beam S1 is caused to have a traveling direction modified to an approximately frontal direction with respect to the prism sheet 4. Precise values of xcfx86a and xcfx86b can be determined in designing under consideration of a peak angle of emission intensity and refractive index of the prism sheet 4, with assistance of Snell""s Law on refraction.
As described above, so far as the main beam, the output light S1 of the prism sheet 4 can be directed to a desired direction (usually an approximately frontal direction) if inclination angles xcfx86a and xcfx86b of the slopes 4a and 4b have appropriate values, respectively.
However, as understood from the graph of FIG. 2, the emission from the emission face 5 includes not only the main beams but also beams which are angularly distributed to some angular range on both sides of the main beams. In the present specification, beams in the left side vicinity of the luminance peak in the graph of FIG. 2 are called a first subsidiary light flux and beams in the right side vicinity of the same luminance peak are called a second subsidiary light flux for the sake of convenience. Further to this, of the first and second subsidiary fluxes are direction-represented by beams which are called a first subsidiary beam and a second subsidiary beam.
As explained above, if the prism sheet is designed under consideration focused on a main beam representing the main light flux, the first and second subsidiary beams are emitted, as a matter of course, toward directions deviated from an output direction of the main beam S1, with the result that some of the subsidiary beams hardly contribute to an illumination operation. In other words, there arises a problem with direction control of subsidiary light flux is subject.
So far as the second subsidiary light flux, if an asymmetric prism sheet disclosed in International Publication WO 98/40664 is employed, much of the second subsidiary light flux can have an output direction similar to that of the main beam.
To the contrary, regarding the first subsidiary light flux, namely, regarding a light flux that is emitted from the emission face 5 toward a direction nearer to the frontal direction as compared with the main beam, the above problem has not been solved and no desirable control of output direction has been achieved.
An object of the present invention is to overcome the above problem. In other words, an object of the present invention is to provide a light control sheet that is able to give an well-controlled output direction to light which is inputted from a direction biased to a frontal direction with respect to a main beam.
Another object of the present invention is to provide a surface light source device that is able to have an well-controlled emission directivity by means of the improved light control sheet. And still another object of the present invention is to provide a liquid crystal display that is able to display an image looking bright as viewed from a certain direction.
The present invention is based on a viewpoint that an input light to a light control sheet, that is used for light-direction-controlling in a surface light source device or the like, can be regarded as a composite light flux generally consisting of a main beam travelling toward a direction inclined with respect to a frontal direction and a subsidiary light flux distributed angularly around the main beam. The present invention improve a light control as to give a well-controlled direction to not only the main beam but also to the subsidiary light flux, applying the improved light control sheet to a surface light source device and liquid crystal display to solve the above problem.
In the first place, the present invention is applied to alight control sheet having a face to provide a light input face and another face to provide a light output face, said light input face being provided with a great number of projection rows running in parallel with each other.
According to an improvement in accordance with the present invention, each of said great number of projection rows includes a first face and a second face, said second face being inclined with respect to a frontal direction of said light control sheet and having a tendency that inclination angle gets smaller with an increasing distance from a top of each of said great number of projection rows.
And said first face is formed so as to introduce an oblique input light containing both a main light flux, which travels in a direction inclined with respect to said frontal direction, and a first subsidiary light flux, which travels in a direction angularly nearer to said frontal direction as compared with said main light flux, and further as to then cause said oblique input light to be inner-incident to said second face.
In a typical embodiment, said second face includes a first region inclined at a first inclination angle with respect to said frontal direction and a second region inclined at a second inclination angle with respect to said frontal direction, said second region being more distant from said top as compared with said first region, said second inclination angle being smaller than said first inclination angle.
In a case where said oblique light further contains a second subsidiary light flux which travels in a direction angularly farther from said frontal direction as compared with said main light flux, the light control sheet is preferably structured so that that said second subsidiary light flux propagates from said first face to said second face and, after being inner-reflected at said second face, is directed to said light output face.
In the next place, the present invention is applied to a surface light source device comprising a light guide plate, a primary light source to supply light to said light guide plate from an end portion of said light guide plate and a light control sheet disposed along an emission face provided by a major face of said light guide plate. Said improved light control sheet is arranged so that said light input face is directed to said light guide plate and a great number of projection rows runs approximately in parallel with an incidence end face provided at said end portion.
From said emission face of said light guide plate supplied is an oblique input light that contains both a main flux travelling toward a direction inclined with respect to a frontal direction of said light control sheet and a first subsidiary flux travelling toward a direction nearer to the frontal direction as compared with said main flux, said input light being introduced into said light control sheet through said first face and then being inner-incident to said second face.
It is noted that a second subsidiary flux travelling toward a direction much deviated from the frontal direction as compared with said main flux is emitted from said emission face and said light control sheet is preferably structured so that the second subsidiary flux travels from said face to said second face, being inner-reflected by said second face toward said first face to be inner-reflected by said first face and directed to said output face.
A surface light source device improved in the above-mentioned manners may be employed for illuminating a liquid crystal display panel of a liquid crystal display. If so employed, the liquid crystal display carries on characteristics of the surface light source device. Therefore a liquid crystal display in accordance with the present invention provides a display screen that looks bright from a certain viewing direction.